FTA (изд-во). Flexography: Principles And Practices. Vol.1-6

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late quality should be assessed

before beginning any printing

job. This section presents a

review of tools and tests devel-

oped to accurately determine

whether or not a plate is press-

ready. Hardness must be determined. Ex-

cess material may need to be trimmed.

Mounting marks might have to be etched,

adhesives may require preparation and ink

formulations may need to be analyzed for

compatiblility. Here is an outline of what

must transpire.

MEASURING PLATE THICKNESS

Plate thickness, or caliper, accuracy is the

most important factor when controlling the

print impression on press. Any low caliper

spots in the plate will either not print or will

cause over-impression of the remainder of

the printing plate. The more accurate the

plate caliper, the longer the plate will last on

press. Plate thickness tolerances of 0.0005"

(12 microns) or better should be expected

on high quality plates.

Plate thickness is measured using special-

ly designed thickness micrometers with

either digital or analog readouts. Standard

thickness micrometers, like those used in

textile and metal fabrication industries, are

unsuitable for measuring flexographic print-

ing plates due to the excessive foot pressure

employed. Flexo plate thickness microme-

ters employ large diameter anvils with very

light application pressure to ensure mini-

mum deflection of the plate material during

measurement.

Bench Micrometers. It is important that the

foot of the micrometer is in full contact with

the section of the plate being measured. The

micrometer must be mounted on a stable

base such as wood, stone or metal to pro-

vide strong support. The base should also be

large enough for the plate to lie completely

flat, allowing the foot of the micrometer to

be in full contact with the plate, thus ensur-

ing more readings.

Since the introduction of large photopoly-

mer plates – sizes from 30" x 80" to 52" x 110"

– bench micrometers are being designed

with bases as large as 54" x 90". It takes prac-

tice to develop proficiency, especially when

using analog micrometers. Only light finger

pressure should be used once the foot is in

contact with the plate.

Analog Indicators. Analog gauges are recog-

nized by the familiar dial indicator face

(Figure

). A revolving pointer rotates

inside a circular scale that represents the

least significant value, for example 0.001".

Each rotation of the pointer represents an

anvil movement of 0.1". A smaller-scaled

PLATES 45

Plate Considerations

A typical analog gauge

used to measure plate

thickness.

46 FLEXOGRAPHY: PRINCIPLES & PRACTICES

Parallax error occurs

when the dial is not

viewed straight on, thus

giving a false reading.

Digital readings avoid

human error such as

parallax error, and is the

most accurate.

Durometer gauges mea-

sures the indentation of

a frustum cone into the

resilient surface under

spring load. The 2"

round style “A” model

durometer gauge is

used for measuring soft,

resilient compounds.

pointer on the face reads the most signifi-

cant value 0.1" increments to 1.000" to keep

track of each revolution.

The term analog, refers to the measuring

scale and how it is presented. Indicated val-

ues are continuously changing and the

slightest change could be significant. When

using an analog dial micrometer it is impor-

tant to be cognizant of, and avoid, parallax

error. This occurs when the dial is not

viewed straight on (Figure

) and can lead

to different interpretations of the same read-

ing. Measurements can also vary from one

person to another if the dial is not perfectly

aligned and the least significant digit needs

to be estimated.

Digital Indicators. When the value can be rep-

resented in digital display, it leaves no room

for interpretation (Figure

), such as the

inherent possibility of parallax error from an

analog gauge. Digital flexo plate micrometers

may be as accurate as 0.0002" or 5 microns,

over its range.

Another feature of digital indicators is the

ability to provide a hard-copy readout in mil-

limeters or inches. A mini-processor and

printer can be attached to record readings

and variances while measuring the plate.

After all the readings are recorded, the

processor can print out the statistical infor-

mation for the particular plate or set of

plates. This information would typically

include the maximum and minimum read-

ings, total variation, average thickness and

average variation. A histogram based on

upper and lower limits set by the user is also

possible. Along with this information, the

job name, date and other data can be printed

and sent to the customer with the plates.

Special interfaces are also available to trans-

mit this information directly to a host com-

puter for maintaining departmental quality-

control statistics and eliminate errors in

reading or incorrectly recorded data.

CHECKING PLATE HARDNESS

The most common instrument for measur-

ing rubber hardness is the Shore durometer

gauge. The hardness gauge measures the

PLATES 47

When trimming plates,

beveling the plate

edges is desirable.

It helps keep the plate

from lifting during the

pressrun.

indentation of a frustum cone into the

resilient surface under spring load. The

durometer hardness scale runs from zero

(softest) to 100 (hardest).

The most widely used Shore durometer

gauge for measuring soft, resilient com-

pounds is the “A” type gauge. The Shore “D”

durometer gauge is used for harder prod-

ucts. The gauges are available in either the

quadrant or round style (Figure

). The

round-style, “A” model, 2" durometer gauge

produces the same readings as the quadrant

type and satisfies the need for an instrument

calibrated in single units rather than in incre-

ments of five units.

For example: For a Shore hardness of 45A,

the stylus would penetrate the surface by

0.055" (0.100"–0.045"). For a proper durome-

ter reading, it is essential that at least a 0.25"

thick sample block of material be used.

Checking a 0.107" thick 50A durometer flexo

printing plate, supported on a hard surface,

would tend to transmit some of the hardness

of the support surface and the polyester

backing sheet, registering a false reading.

Temperature is also a critical factor when

measuring hardness, especially of pho-

topolymers. Hot material will produce a soft-

er reading.

Time is another factor to be considered

with most plate materials. As the stylus pen-

etrates the surface of the plate material, the

hardness reading will drift downwards. Both

instantaneous readings and readings taken

after five seconds of gauge application,

should be compared. Hardness readings

must not be taken on the face of the printing

plate because this may cause irreparable

damage. The durometer gauge should be

handled with care, and checked and recali-

brated if necessary.

CARE AND HANDLING OF PLATES

Whether rubber or photopolymer, finished

plates must be trimmed accurately. The pre-

ferred method of trimming a printing plate is

to do so using a plate cutter or foot shear. If

a knife is used to trim photopolymer plates,

the cut should be made from the polyester

backing side of the plate. Edges must be

smooth and free of nicks and burrs.

Beveling all sides of the plate is desirable

and may be done by securing a rigid material

on the bed of the paper cutter or foot shear

(Figure

). Place the finished plate on top

of the rigid material and press firmly to trim.

This flexes the edge of the plate and cuts a

neatly beveled edge. The position and height

of the rigid material determines the bevel

angle. The beveled plate edges help keep the

plate from lifting during the pressrun.

PLATE MOUNTING

Centerlines may be drawn on the floor of

plates with a ballpoint pen or lightly in-

scribed on the reverse side of the polyester

backing of photopolymer plates with a film

cutter. With the center lines permanently

marked on the plate, white or orange mount-

ing chalk rubbed over the lines make them

more visible during mounting on an optical

mounting machine. Register marks imaged

on the plate are usually left in place during

mounting and throughout press makeready.

They can be removed before the production

Plate-Edge Profile

Feed Board

Hold

Down

Cutting

Knife

48 FLEXOGRAPHY: PRINCIPLES & PRACTICES

run using any sharp cutting instrument. Care

should be taken to avoid damaging any

image area during this process.

Precurving Plates. When plates are to be

mounted on small diameter cylinders, it is

recommended that the plate be precurved.

This is done to prevent lifting during the

pressrun. The precurving procedure relaxes

the plate and increases conformity to the

curvature of the plate cylinder. Plates are

precurved by heating the plate in a tempera-

ture-controlled oven (plate dryer) to 140° F

(60° C) for 10 to 15 minutes. The warm plate

is then covered with a piece of polyester slip

sheet and rolled in the print direction with

the print face outward to approximately the

print cylinder size (Figure

). The rolled

plate is then allowed to cool to room tem-

perature for at least four hours.

Mounting Tapes (Stickyback). Plates may be

mounted with any commercially available

double-sided tape called stickyback. High-

tack tapes are recommended for mounting

photopolymer plates, especially on small

diameter cylinders. Tapes should be of uni-

form thickness to get the most out of gauge-

controlled platemaking. Often, “highs and

lows” in printing are mistakenly blamed on

the plate, when they are really caused by

uneven mounting tape or air entrapment.

There are many types of tapes available and

tape should be chosen based on its tack and

cushion properties for the job being printed.

Edge Sealing. Plate-edge sealant will prevent

ink and solvents from attacking the adhesive

tape during printing and plate washup. After

the plates have been set onto the adhesive

tape, the edge seal should be applied in a

fine bead around the plate border. The

sealant must be allowed to dry thoroughly

before wrapping cylinders to continue set-

ting of plates onto the adhesive.

Demounting Plates. If plates are to be stored

and reused, care should be taken when

demounting from the double-sided mount-

ing tape. Rubber plates can be stretched or

torn, and photopolymer plates may be sus-

ceptible to delamination from their poly-

ester backing or kinking of the polyester

backing. Special medium-tack double-sided

mounting tape is available for de-mountable

applications.

PLATE WASHUP

Proper plate washup on press can length-

en plate life. Plates should be washed imme-

diately after printing with the correct plate-

wash solution before the ink has time to set.

Plate manufacturers’ suggestions should be

followed when determining which plate

wash to use. Plates should never be

scrubbed using a wire- or stiff-bristled brush.

Ample quantities of the correct solvent, or

prepared plate wash, should be used in con-

junction with a lint-free cloth that will cut

the ink without hurting the plate material.

The plate should be swabbed gently until the

ink loosens and can be sponged off with a

second cloth. Final drying may be achieved

using a soft, absorbent paper. Forced air

may also be used to blow away the residual

solvent and lint.

Note: When cleaning plates on the cylin-

ders, care should be taken not to let the sol-

vent or cleaning agent get under the sticky-

back. That can cause the plate to lift from

Precurving plates

relaxes the plate and

increases conformity to

the curvature of the

plate cylinder, thereby

preventing plate lift

during the pressrun.

the cylinder or off the stickyback itself.

Once the plates are clean and dry, they

should be dusted with talc or fine corn

starch and placed in a proper storage area.

PLATE STORAGE

The temperature in plate storage areas

should not exceed 100° F (38° C). The plate

storage area should also be located away

from ozone sources such as power stations,

press-motor drives and corona-discharge

film-treating units. Plates should be kept in

a cool, dry, dust-free environment, away

from direct exposure to light sources.

Plates stored on cylinders are more sus-

ceptible to deterioration from ozone attack.

If plates must be stored in this way, they

should be thoroughly cleaned and dried,

then tightly wrapped in black polyethylene

to protect against ozone exposure.

When high ozone levels cannot be avoid-

ed, applying ozone-resistant finishes to the

cleaned and dried plates may provide some

protection. Consult your materials supplier

for recommended materials.

PLATES 49

Table 11. Reprinted with permission from Fulfex, Inc.

SOLVENT NATURAL RUBBER BUNA “N” ETHYLENE PROPYLENE

(EP)

Acetone F NR S

Benzene NR NR NR

Carbon Tetrachloride NR NR NR

Cellosolve S F S

Cellosolve Acetate F NR S

Ethyl Acetate NR NR F

Ethyl Alcohol S S S

Isopropyl Acetate NR NR F

Isopropyl Alcohol S F S

Kerosene NR S NR

Lactol Spirits NR S NR

Methyl Alcohol S S S

Methyl Ethyl Ketone NR NR S

Methyl Isobutyl Ketone NR NR S

Mineral Spirits NR S NR

Naptha VMP NR S S

Normal Butyl Acetate NR NR F

Normal Propy Alcohol S S S

Toluene NR NR NR

Xylene NR NR NR

S Satisfactory

F Fair

NR Not Recommended

Note: Guidelines only; to ensure compatibilty, contact supplier or conduct swell test.

RUBBER PLATE AND SOLVENT COMPATIBILITY

50 FLEXOGRAPHY: PRINCIPLES & PRACTICES

INK AND SOLVENT COMPATIBILITY

A simple test for swelling can be used to

determine the relative compatibility be-

tween the plate material and printing solu-

tion. Take a small section of a fully cured

plate material, measure the thickness, and

immerse it for 24 hours in the solution to be

tested. Remove the sample from the solution

and, after blotting the sample dry, again mea-

sure the thickness. If the material has gained

more than 5% in gauge, then the two materi-

als may be considered incompatible. For

thinner plates (0.045" or 0.067"), this means a

value of 2 to 3 mils and for a thick plate

(0.25") a value of 10 to 12 mils.

A more complete test is to leave the sam-

ple out of the solvent for 24 hours and then

re-immerse it for an additional 24 hours. In

most cases, the abbreviated single 24-hour

test will indicate if the plate and solvent are

compatible. A longer test may be warranted

on a plate which will be used for a long run

or for repeated runs.

Table 11 lists the solvent compatibility for

rubber plates. Table 12 lists the compatibili-

ty for photopolymer plates.

Table 12. Reprinted with permission from E .I. duPont de Nemours and Company.

MAXIMUM % IN

NORMAL PROPYL MAXIMUM % IN

PURE SOLVENT ALCOHOL COSOLVENT WATER COSOLVENT

PHOTOPOLYMER PLATE AND SOLVENT COMPATIBILITY

KETONES

Acetone N 5 5

Methyl Ethyl Ketone N 5 5

Methyl Isobutyl Ketone N 5 5

ALIPHATIC/AROMATIC HYDROCARBONS

1,2

Heptane N 5 n/a

Hexane N 5 n/a

Cyclohexane N — n/a

VM&P Naptha (3% aromatic) N 5 n/a

Lactol Spirits 9300 (9% aromatic) N 5 n/a

Lactol Spirits 9500 (14% aromatic) N 3 n/a

Lactol Spirits 45 (19% aromatic) N 3 n/a

Lactol Spirits 50 (32% aromatic) N 3 n/a

Benzene N 1 n/a

Toluene N 1 n/a

Xylene N 1 n/a

Ethyl Benzene N 1 n/a

GLYCOL ETHERS

Butyl Cellosolve N 3 3

Ethyl Cellosolve N 30 30

Proposal P N 30 30

Carbitol N 30 30

cont’d on the following page

For extended run lengths, lower maximum percentages are recommended for best results.

This category includes petroleum/paraffinic distillates.

WRAP DISTORTION

Elastomeric printing plates that are made

in the flat and then wrapped around a print-

ing cylinder experience distortion (elonga-

tion) in print length. The amount of distortion

depends on the dimensions of the plate and

the cylinder, as well as plate construction.

Photopolymer plates tend to have greater,

but more constant distortion factors than

their rubber counterparts. This is due to the

dimensionally stable polyester backing sheet.

Two factors determine image elongation in

printing plates:

1.The thickness of the elastomeric layer

above the neutral plane. The neutral

plane on polyester-backed plates is dis-

placed to a point just above the poly-

ester carrier. In nonbacked rubber

plates, the neutral plane runs through

the center of the plate thickness.

2.The diameter of the printing cylinder.

Distortion-correction factors for polyester-

backed photopolymer plates up to 250 mils

thick and for cylinder sizes up to 60" repeat

length are shown in Figures

PLATES 51

Table 12. Reprinted with permission from E .I. duPont de Nemours and Company.

MAXIMUM % IN

NORMAL PROPYL MAXIMUM % IN

PURE SOLVENT ALCOHOL COSOLVENT WATER COSOLVENT

Water Y 100 —

AMINES (PH ADJUST)

28% Ammonium Hydroxide Y n/a —

2-Amino-2-Methyl-1-Propanol Y n/a —

Morpholine N n/a —

Monoethanol Amine Y n/a —

Triethanol Amine Y n/a —

ALCOHOL/GLYCOLS

Methyl Alcohol N 50 50

Ethyl Alcohol Y 100 —

Isopropyl Alcohol Y 100 100

Normal Propyl Alcohol Y — 100

Normal Butyl Alcohol Y 100 100

Octyl Alcohol N 5 5

Benzyl Alcohol N 5 5

Ethylene Glycol Y 100 100

Propylene Glycol Y 100 100

Diethylene Glycol Y 100 100

Dipropylene Glycol Y 100 100

Triethylene Glycol Y 100 100

Glycerine Y 100 100

ESTERS

Ethyl Acetate N 20 n/a

Isopropyl Acetate N 20 n/a

Normal Propyl Acetate N 20 n/a

PHOTOPOLYMER PLATE AND SOLVENT COMPATIBILITY CONT’D

52 FLEXOGRAPHY: PRINCIPLES & PRACTICES

The calculations are based on the formula

for distortion:

DCF  1  2(Tp  Tb)

Where:

DCF = Distortion correction factor

R = Printing circumference

(repeat length) of cylinder

Tp = Plate thickness (inches)

Tb = Thickness of the polyester

backing sheet

Note: Figure

is calculated with a Tb

value of 0.007". On the scale of the figure,

using a different Tb value, such as 0.004",

would show no significant difference.

A second formula commonly used to

determine the percent a film negative must

be reduced in order to compensate for image

distortion is:

% reduction 

 100

Where:

K = a constant supplied by the plate

material manufacturer

R = Printing circumference (repeat

length) of cylinder (inches)

Notice that K is equal to 2 (TpTb) in the

DCF calculation. Table 13 lists calculated K

factors for common plate thicknesses with

0.004" and 0.007" backing. The K factor

depends on the measurement system used.

The table lists the value for repeat lengths in

inches and centimeters.

Example: What is the distortion needed in

The graph at right plots

the alculated distortion-

correction factors for

polyester-backed pho-

topolymer plates up to

0.25" thock and repeat

lengths up to 60".

Plate Thickness

1.00

.250

.187

.125

.100

.067

.155

.045

.030

0.95

0.90

0.85

0.80

0.75

0.70

5 1015202530354045505560

Cylinder Size = Repeat Length

Distortion Correction Factor (DCF) %

film negatives for a 0.067" plate with a

0.004" backing sheet and a repeat length of

8"?

From Figure

, the distortion factor is

about 0.95. Using the K-value calculation,

the percent reduction is 0.396 (from Table

13) divided by 8, times 100. This gives a

value of 4.95%. The distortion factor would

be 95.05%. This is the same as the 0.95 (95%)

from Figure

. Clearly, Figure

only in-

dicates a rough value for the distortion fac-

tor. For precise values, the percent reduc-

tion or DCF formula should be used.

In principle, distortion factors could be

calculated for rubber plates also. Since rub-

ber plates have the shrink as well as wrap

distortion and are unbacked, the distortion

is usually determined empirically.

Surface Tension

Surface tension is a condition existing at

the free surface of a liquid, resembling the

properties of an elastic skin under tension.

Dynes per centimeter is the unit that is gen-

erally used to measure surface tension. One

dyne is the force one milligram exerts under

the influence of gravity. Printing plates, sub-

strates, and inks have a dyne value. A practi-

cal example of what dyne and surface ten-

sion is all about can be seen in the reaction

of water on a waxed surface. Plain water will

bead up on a waxed surface because the sur-

face tension of the water is greater than that

of the wax. If a surfactant, such as detergent

or alcohol, is added to the water to lower the

surface tension, it will spread and wet the

wax surface. This is known as wetting out.

Surface energy and its relation to ink

transfer and printability is not understood

well enough to allow exact use of surface-

energy specifications for plates, inks, trans-

fer rollers or anilox rollers.

The surface energy values for water-based

inks are between 34 to 38 dynes/cm, while

the values of resins used in solvent-based

inks are 28 to 32 dynes/ cm.

Most photopolymer plate materials have

lower, but more consistent, surface energy

than natural rubber. Materials with higher

PLATES 53

Table 13

INCHES CENTIMETERS

PLATE K FACTOR PLATE K FACTOR

THICKNESS 0.004 BACKING 0.007 BACKING THICKNESS 0.004 BACKING 0.007 BACKING

0.030 0.163 0.145 0.076 0.415 0.367

0.045 0.258 0.239 0.114 0.654 0.606

0.067 0.396 0.377 0.170 1.005 0.958

0.080 0.478 0.459 0.203 1.213 1.165

0.090 0.540 0.522 0.229 1.372 1.325

0.100 0.603 0.584 0.254 1.532 1.484

0.107 0.647 0.628 0.272 1.644 1.596

0.112 0.679 0.660 0.284 1.724 1.676

0.125 0.760 0.741 0.318 1.931 1.883

0.155 0.949 0.930 0.394 2.410 2.362

0.187 1.150 1.131 0.475 2.921 2.873

0.250 1.546 1.527 0.635 3.926 3.878

K FACTORS

54 FLEXOGRAPHY: PRINCIPLES & PRACTICES

surface energy have a greater affinity for flu-

ids with lower surface energy. Consequently,

natural rubber plates, with higher surface

energy, accept ink more readily from the

anilox roll than photopolymer plates.

Photopolymers exhibit high critical sur-

face tension values. Ink wetting and transfer

properties increase as plate wettability

increases. Plate wettability increases as the

critical surface tension of the plate exceeds

that of the ink. Surface energy of ink, rollers

and substrates can sometimes be altered

within narrow limits to affect the amount of

ink film transferred by the printing plates.

All these factors have a direct bearing on the

final ink transfer. See also the section per-

taining to the dyne level of substrates in the

ink volume.